I. Field of the Invention
The present invention concerns transdermal delivery of therapeutic agents by use of an applied electromotive force, commonly known as iontophoresis. More particularly, the invention is directed to a system for iontophoresis that is self contained, quantitatively self limiting, and in which the delivery rate is variable or adjustable. The system is contained preferably in a rather small skin worn patch which contains electrodes and a therapeutic agent. When applied to the skin, the system completes a circuit and spontaneously initiates the flow of a galvanic current of measured, limited duration corresponding to the desired amount of therapeutic agent to be delivered. The system may be anode or cathode limited.
II. Related Art
The process of iontophoresis was described by LeDuc in 1908, and has since found commercial use in the delivery of ionically charged compounds such as pilocarpine, dexamethasone, and lidocaine. In this delivery method, ions bearing a positive charge are driven across the skin at the site of an electrolytic electrical system anode, while ions bearing a negative charge are driven across the skin at the site of an electrolytic electrical system cathode.
With iontophoretic devices, the application time and level of current flow (usually reported in units of milliamp-minutes) between the anode and cathode is directly correlated to the amount of drug delivered. The efficiency of drug delivery in an iontophoretic system can be measured by the proportion of current carried by the drug molecule, relative to the current carried by competing non-medication ions having the same charge as the medication.
At present, iontophoresis devices are conventionally comprised of two electrodes attached to a patient, each connected via a wire to a microprocessor controlled electrical instrument. Medication is placed under one or both of the electrodes, for delivery into the body as the instrument is activated. The instrument is designed to regulate current flow and application time. Examples of such instruments are described in U.S. Pat. Nos. 5,254,081, and 5,431,625. Power for these devices is usually provided by DC batteries, which when providing power for the microprocessor controlled circuitry allow application of a voltage to the electrodes to create a regulated current flow. The automated control of current flow and time (milliamp-minutes) is of great advantage, in order to prevent excessive dosages of therapeutic agents from being delivered. However, these battery powered microprocessor systems are disadvantaged by the fact that patients are xe2x80x98attached by wirexe2x80x99 to an instrument, which limits patient mobility and ability to conduct normal daily activities. A typical application period is approximately 20 minutes to 2 hours, which consumes instrument, caregiver, and patient time.
A significant advantage of a microprocessor controlled iontophoretic system is an ability to adjust electrical current as a function of time, while the system is being used. For example, to administer medication quickly into systemic circulation, an initial high flow rate is desired. However, adjustment to a lower flow rate may be desired afterward for optimal maintenance of a particular plasma medication level.
More recently, wearable iontophoretic systems have been developed in which the electrical circuitry and power supplied are integrated into a single patch. These systems are advantageous in that they do not require external wires, and they are much smaller in size. Examples of such systems can be found in U.S. Pat. Nos. 5,358,483; 5,458,569; 5,466,217; 5,605,536; and 5,651,768. However, these systems also have drawbacks. They are relatively inflexible and expensive, owing to the requirements of multiple electronic components, battery power supplies and electrical interconnects.
Power to drive iontophoretic current flow can also be supplied by galvanic means, which utilizes dissimilar anode and cathode materials to produce a spontaneous current flow when they are contacted with the body. These systems hold advantage, in that separate electrical circuitry and battery sources are not required. An iontophoretic device, not of the transdermal type, but one which utilizes galvanic means is described in U.S. Pat. No. 5,322,520, which describes an implanted device designed to deliver oligodynamic metal ions from its surface, in order to kill bacteria on or near it.
Devices suggesting galvanic power as a means to transdermally deliver medication are described in U.S. Pat. Nos. 5,162,042, and 5,405,317. These devices are disadvantaged by the fact that the amount of medication delivered is not automatically regulated, and they require a timely removal of the device from the body to prevent a potentially toxic over-dosage of medication.
In a co-pending application PCT/US99/18861, designating the U.S., claiming priority based on U.S. provisional application No. 60/098,652, assigned to the same Assignee as the present application, an iontophoresis patch system is described which uses galvanic power and provides a known dosage capacity. Thus, this system can be designed to automatically shut off after a specified dosage, and the risk of overdosage is eliminated. That co-pending application is deemed incorporated herein by reference for any purpose.
Horstmann, in U.S. Pat. No. 5,685,837, describes a transdermal therapeutic system which uses series mounted sheet-like galvanic elements as a power source. That device has an ability to either create a constant intensity of current using a high internal resistance element, or create a gradual decreasing current intensity using a low internal resistance element. The low internal resistance, and a decreasing current flow, is caused by a build up of ions into an electrolyte layer of the galvanic element.
While it appears advantageous, there are certain practical disadvantages to the Horstmann system. To achieve a decreasing current, a very thin electrolyte layer required. The thin layer is susceptible to mechanical failure during production or use. Also, rather than the gradual decreasing current of the Horstmann system, a steady high rate of current which then rapidly falls is optimal, so that a known charge dosage can be administered in minimal time. The actual charge dosage administered using the Horstmann device is not accurately known, since a nernstian decline in voltage is in a non-linear diminishing rate, and thus the system will not fall to zero current during practical time scales.
One restriction of all galvanic systems is a limited supply of user-friendly materials which can be practically used. Many materials may be toxic themselves, and/or they may be difficult to work with in the manufacturing process. A significant problem lies with materials that are reactive with water, and therefore can alter the pH of the medication solution during use. pH changes can harm skin, or cause adverse reaction with medication. For example, we have found zinc (E0=xe2x88x922.37) to be an excellent galvanic material, but magnesium (E0=xe2x88x922.37) causes a pH change in iontophoretic medication chambers. Silver chloride (E0=+0.222) does not affect pH of a medication chamber, but manganese dioxide (E0=+1.23) does. Consequently, the voltages obtainable in galvanically powered systems are limited by material stability or compatibility. Of course, various other species may have application as oxidizable or reducible species under different circumstances.
Another restriction or limitation of galvanically powered systems is an inability to increase or decrease voltage (and medication delivery) during use. The voltage is fixed by the galvanic xc2xd reactions used, and cannot be altered in process. This is a significant disadvantage in circumstances where increasing rate of delivery, such as to administer a bolus of medication, is desired. Accordingly, it would present a great advantage were increased potential available in such a device.
A primary object of the invention is to provide a galvanically powered iontophoretic device which can provide any desired time-voltage profile (and consequently customize the rate profile of medication delivery) using materials which are non-pH reactive in contact with water, and stable when used in the in a manufacturing process.
A further object of the present invention is to provide an iontophoretic device which can provide any desired time-voltage or delivery profile using circuit components without requiring a microprocessor.
Another object of the invention is to provide an iontophoretic device capable of maintaining voltage at a stable level for a known charge dosage, afterwards having an automated ability to adjust the voltage downward or upward to at least a second known voltage in rapid fashion, without requiring an integrated microprocessor.
It is a still further object of the invention to provide a galvanically powered system which can be adjusted to higher voltage during use, in order to administer a bolus of medication.
Still another object of the invention is to provide a galvanically powered system in which the potential level can be made time variable by employing several serially connected galvanic sources of different coulombic capacities in conjunction with parallel connected resistor devices.
Other objects and advantages will occur to those skilled in the art upon familiarization with this specification, drawings and appended claims.
An iontophoretic patch in accordance with the invention includes in its simplest form at least three chambers: a cationic drug chamber, an anionic drug chamber, and an additional galvanic source or cell. The system may also have a series of such cells together with one or more parallel connected resistance devices or other components in more complex versions. In the cationic drug chamber, an electrode is contained which includes or is coated with, an electrochemically oxidizable species. In the anionic drug chamber, an electrode is contained which includes or is coated with, an electrochemically reducible species. The chambers are separated by a known distance optimally between 0.1 and 2 cm. The cationic chamber electrode and the anionic chamber electrode are connected by electrically conductive elements to an additional intermediate electrochemical cell or cells. Opposing ends of the electrically conductive elements, which extend to the intermediate cell or cells, are comprised of, or coated with, reductive species if the end opposite is oxidative, or oxidative species if the end opposite is reductive.
The net result is a series connection of galvanic couples or sources, which serve to boost the applied potential of the iontophoretic system. Embodiments employing an illustration of an iontophoretic system using zinc as the oxidative species and silver chloride as the reducible species, having one intermediate chamber or galvanic source and providing an applied potential of approximately 2 volts and also using two intermediate chambers or galvanic sources and providing an applied potential of approximately 3 volts are shown in the drawings to illustrate the concept without limitation. The total applied potential of the galvanic system of this invention is directly correlated to the number of intermediate chambers utilized. It will be apparent that the number of intermediate galvanic sources or couples can be varied depending on the desired total potential.
In addition, one or more resistor devices can be connected in parallel with one or more of the galvanic sources. This assures serial operation of multiple source systems while all sources are operating and provides paths across depleted cells of lesser capacity in the system so that a therapeutic agent can continue to be administered to a lower rate.
In addition, one or more switching devices can be provided in the iontophoretic patch to switch one or more of the intermediate sources or chambers into the circuit or to bypass or shunt one or more galvanic sources as desired. A switching device rather than a resister can also be used to by-pass a depleted galvanic source without added circuit resistance if it is desired to provide an initial bolus of therapeutic material, all serially connected sources or cells can be employed for an initial period and one or more configured to deplete and thereby reduce the potential at a later time.
Thus, one or more of the intermediate chambers can be made anode or cathode limited in a manner which causes depletion at a time when the desired bolus of initial material has been delivered. Parallel resistors or switches, then, provide a conductive path to enable delivery to continue at a lower rate. Switching devices can also be employed which shut off the entire galvanic flow in the iontophoretic patch or to switch in additional series connected galvanic sources as by opening by-pass conductor circuits for example.
In yet another type of configuration, galvanic sources can be serially connected in opposed polarity such that a lower capacity source opposes a therapeutic or agent delivery source to initially delay agent delivery until the lower capacity source is depleted.
The oxidizable species and the reducible species are selected so as to provide a spontaneous galvanic potential and current flow when the iontophoretic patch is in contact with the body. An example of a suitable oxidizable species is zinc and a suitable reducible species is silver chloride. While this combination may be advantageous for many applications, it is not meant as a limitation as the scope of the invention but is presented only by way of example.
During the iontophoretic process of this invention, as current flows, the oxidizable species in the cationic drug chamber and one or more intermediate cells become oxidized, while the reducible species in the anionic chamber and intermediate cells become reduced. The galvanically induced current will continue to flow until depletion of either the oxidizable or reducible species, whichever is present in limiting amount. The relationship between the amount of current flow and the amount of oxidizable or reducible species in limiting supply, is theoretically represented by Faradays constant; one gram equivalent of the limiting reducible or oxidizable species will provide one Faraday (96,487 coulombs) of electricity. The iontophoretic patch of this invention will optimally deliver a fixed and known charge in a range between about 0.06 and 60 coulombs, which corresponds to between 0.00000062 and 0.00062 gram equivalent weight of oxidizable or reducible species in limiting supply.
Preparation of the iontophoretic electrodes of this invention is critical, as a known limiting amount of electroactive species must be incorporated within, or onto, any anode electrode, any cathode electrode, or multiple electrodes. In preparation of the cationic drug chamber electrode, oxidizable material can be used of known weight and purity; or an oxidizable coating of known amount can be deposited on the surface of an electrically conductive substrate. For example, a known amount of molten zinc can be deposited over a wire substrate, to produce an electrode with known oxidizable species content. In preparation of the anionic drug chamber electrode, a reducible material of known amount can be deposited on the surface of an electrically conductive substrate. For example, a known amount of molten silver chloride can be deposited over a wire substrate, to produce an electrode with known reducible species content. Alternatively, a known amount of silver chloride can be generated on the electrode surface by an electrolytic or electroplating process, such as by electrolytic oxidation of a silver wire in the presence of chloride, to produce a coating of silver chloride.
The preferred approach to preparation of the iontophoretic electrodes of this invention is by screen printing of thin coatings, having known amounts of electroactive materials, over a conductive trace on a flexible substrate. This process yields a controlled dosage galvanic battery, for incorporation into the iontophoretic patch. Other circuit elements may also, be created using thin film, screen printing or other such methods in accordance with state-of-the-art techniques.
Other aspects of the patch include an impervious backing material, which can be constructed with 3M polyethylene tape #1523, #1526, (available from 3M Corporation, St. Paul, Minn.) or other occlusive material. Holding the electrodes in place, and attached to the backing material is a cell wall defining layer, which has separated openings to define anode and cathode cell cavities, as well as intermediate chamber cavities. The cell wall defining layer can be constructed of 3M #1772 or similar material. An absorbent layer is added to each of the cavities defined by the cell wall defining layer, and serves to retain fluid in the cell cavity. The absorbent layer can be a material which forms a gel when contacted with aqueous solution such as polyacrylamide, or it can be cotton, gauze, or other hydrophilic material. An adhesive layer is used to fix the iontophoretic device to the skin, which can also be comprised of materials such as 3M #1523 or #1526.
To use the controlled dosage iontophoresing device, solution containing cation to be delivered is put into the cationic drug chamber, and solution containing anion material is injected into the anionic drug chamber. The intermediate chamber is preferably filled with a conductive salt solution, through an access port on either the top or bottom of the device. Optionally, the intermediate chamber can be pre-filled with a salt-containing gel or paste. Unlike the Anionic and Cationic Drug Chambers, the intermediate chamber or source or sources should not be in contact with the skin. The patch is then applied to the portion of the body where drug is to be administered, and adhered to the skin by an adhesive layer on the bottom of the patch and or by an overlaying bandage material. Once contacted with skin, an electrical circuit is completed which allows passage of current and delivery of drug compounds.